The latest OS 1.01 can be downloaded from http://www.korg.com/SupportResults.aspx?productid=179 under "Downloads"

It also has a note on it
"Note: Do not try to load this Operative System in instruments other than Pa50SD"

which pretty much answers Jacob's question. Trying to load an incompatible OS could very likely turn the Microarranger into a useless brick.